1. Field of the Invention
The present invention relates to a full charge control apparatus installed on a vehicle for controlling an onboard battery which is charged by electric power supplied from a generator driven by an engine.
2. Description of the Background Art
It is possible to increase the capability of acceptance of regenerative energy of a secondary battery while controlling a state of charge (SOC) thereof to a level lower than a full charge capacity (nominal capacity) of the battery. It is known however that this kind of control operation, which is hereinafter referred to as partial-state-of-charge (PSOC) control operation, has one drawback in that the control operation causes the battery to deteriorate. In particular, if the battery is operated under conditions where the state of charge is lower than a tolerable SOC range of the battery, a phenomenon like sulfation which will accelerate deterioration of the battery may occur. To successfully perform the PSOC control operation, it is needed to determine the state of charge of the battery with high accuracy and operate the battery in such a way that the state of charge does not become lower than a lower limit of the tolerable SOC range of the battery.
Methods usable for determining the state of charge of a battery include a current integration method and an open circuit voltage (OCV)-SOC method, for instance. In the current integration method, the battery is fully charged at regular intervals and charging and discharging currents are integrated from each point in time of full charge to determine the state of charge of the battery. Operation for controlling the battery to reach a fully charged state (i.e., full charge control operation) would be to charge the battery up to a point where the following conditions are met: “the charging current becomes equal to or less than a specific value and, in addition, a specific period of time elapses.” A method of determining a charge capacity of the battery from current-SOC maps prepared in advance for different battery temperatures is known in the prior art and it is often the case that the operation for bringing the battery to the fully charged state is executed immediately after engine start.
If a charging current to be used for judgment of the fully charged state is set at the same level regardless of temperatures, it is necessary to detect the battery temperature with high accuracy. This is because the state of charge of the battery differs with the battery temperature. In many cases, however, the battery temperature is determined by detecting an ambient temperature of the battery which may potentially differ from a true battery temperature. Thus, it is not usually expected that the battery temperature is detected with high accuracy. In a case where the state of charge of the battery is determined from the detected temperature by using the current-SOC maps prepared for different temperatures, there is a possibility that the state of charge of the battery determined may differ from the actual state of charge if temperature detection accuracy is not high. As a result, if the state of charge at the beginning of SOC calculation, or the state of charge under conditions where the full charge control operation has been executed, is lower than an expected value (e.g., SOC=90% or less), there is such a likelihood that the battery could reach a region in which deterioration of the battery is accelerated.
Under such circumstances, Japanese Laid-open Utility Model Publication No. 1994-84754 proposes a full charge control apparatus for a secondary battery that detects both the temperature of the battery and the ambient temperature thereof and performs the full charge control operation in such a manner that the battery is not wrongly judged to have been fully charged even when the ambient temperature is varying.
Particularly when the state of charge of an onboard battery which is mounted close to a vehicle engine, for instance, is determined from current-SOC maps prepared in advance for different battery temperatures as in conventional cases, there can be a case where the accuracy of detection of the battery temperature deteriorates due to variations in surrounding conditions of the battery according to engine operating conditions. In a case where the state of charge of the battery has become lower than the expected value, there is a possibility that the battery becomes insufficiently charged. Thus, if the battery is operated in the state of charge that is lower than the tolerable SOC range, deterioration of the battery may be accelerated by sulfation, for instance, as mentioned above.
Incidentally, there are cases where the ambient temperature of the battery is affected by the battery temperature. Therefore, if there are transient variations in the surrounding temperature, the battery temperature may rapidly increase near the fully charged state as mentioned in Japanese Laid-open Utility Model Publication No. 1994-84754 cited above. For this reason, the accuracy of battery temperature detection may potentially deteriorate when the difference between the battery temperature and the ambient temperature increases.